(1) Field of the Invention
The present invention relates to an electron emission element and a method of manufacturing the same and, more particularly, to an electron emission element for causing an avalanche breakdown to externally emit hot electrons, and a method of manufacturing the same.
(2) Related Background Art
As a conventional electron emission element, many kinds of cold cathode electron emission elements have been studied. An electron emission element using a semiconductor material will be exemplified below as a conventional electron emission element.
Electron emission elements undergo various improvements along with the progress of semiconductor techniques.
As electron emission elements using a semiconductor material, for example, an element for applying a forward bias to a p-n junction by utilizing a negative electrode affinity to emit electrons (Japanese Patent Publication No. 60-57173), an element for applying a reverse bias to a p-n junction to cause an avalanche breakdown and emitting electrons produced by the avalanche breakdown (U.S. Pat. Nos. 4,259,678 and 4,303,930), and the like are known.
Of the conventional electron emission elements, an element employing an avalanche breakdown is arranged as follows, as described in U.S. Pat. Nos. 4,259,678 and 4,303,930. That is, p- and n-type semiconductor layers are joined to constitute a diode structure. A reverse bias voltage is applied across the diode to cause an avalanche breakdown, thereby producing hot electrons. The electrons are emitted from the surface of the n-type semiconductor layer on which cesium or the like is deposited to reduce the work function of the surface.
The surface layer of each conventional electron emission element comprises a single electrode layer.
A technique for reducing the work function of an electron emission surface to improve electron emission efficiency is known in association with these conventional electron emission elements. For example, in an electron emission element in which a reverse bias is applied to a p-n junction to cause an avalanche breakdown, cesium or the like is deposited on the surface of an n-type semiconductor layer to reduce the work function, thereby improving electron emission efficiency.
A Schottky electron emission element structure known to applicants is e.g., FIG. 1. In FIG. 1, a p.sup.- -type GaAs layer 102 as a semiconductor layer is formed on a p.sup.+ -type GaAs substrate 101 as a semiconductor substrate by, e.g., molecular beam epitaxy (MBE). A p.sup.+ -type region as a high-impurity concentration region 103 for causing an avalanche breakdown is formed in the semiconductor layer 102 by implanting Be ions. An element isolation insulating layer 104 and a wiring electrode 105 are formed on the semiconductor layer 102, and a Schottky electrode 108 of, e.g., tungsten is also formed on the layer 102 by, e.g., sputtering. A lead electrode 107 is formed on the wiring electrode 105 via an insulating layer 106 of, e.g., SiO.sub.2. An ohmic electrode (110) formed on the substrate (101) and an extraction electrode (111) is formed on the lead electrode (107).
The Schottky electron emission element shown in FIG. 1 is manufactured as follows. That is, the high-impurity concentration region 103 is formed in the semiconductor layer 102 by, e.g., ion implantation, and the resultant structure is subjected to proper annealing. Thereafter, a conductive layer is formed on the resultant structure and is patterned, thereby forming wiring electrodes 105. Thereafter, the insulating layer 106 is formed, and a hole is formed. Finally, a conductive layer is formed and patterned to form the Schottky electrode 108.
However, when the conventional electron emission element employs a p-n junction type diode structure, switching characteristics of the element are much lower than that of a Schottky diode, and the upper limit of a direct modulation frequency of the electron emission element is low. Therefore, applications using the electron emission element tend to be limited to a narrow range.
The conventional electron emission element has a guard ring structure around an electron emission section. However, in order to form the guard ring structure, a large element area is required, and it is difficult to achieve higher integration and micropatterning of the element.
Furthermore, the conventional electron emission element suffers from complex processes for forming an n-type guard ring layer, a p-type high-concentration layer, and an n-type surface layer on a p-type semiconductor layer, and also suffers from a technical difficulty for forming a very thin doped layer, resulting in a poor manufacturing yield. Therefore, manufacturing cost tends to be increased.
When cesium or a cesium oxide is formed on the surface of the electron emission section to reduce the work function of the electron emission section, since the cesium material is chemically very active, the following problems are always posed:
(1) a stable operation cannot be expected unless it is used in ultrahigh vacuum (10.sup.-7 Torr or higher); PA1 (2) a service life is changed according to a degree of vacuum; and PA1 (3) efficiency is changed according to a degree of vacuum. PA1 a semiconductor substrate having a p-type semiconductor layer whose impurity concentration falls within a concentration range for causing an avalanche breakdown in at least a portion of a surface thereof, PA1 a Schottky electrode for forming a Schottky junction with the p-type semiconductor layer, PA1 means for applying a reverse bias voltage to the Schottky electrode and the p-type semiconductor layer to cause the Schottky electrode to emit electrons, and PA1 a lead electrode, formed at a proper position, for externally guiding the emitted electrons, PA1 wherein at least a portion of the Schottky electrode is formed of a thin film of a material selected from the group consisting of metals of Group 1A, Group 2A, Group 3A, and lanthanoids, metal silicides of Group 1A, Group 2A, Group 3A, and lanthanoids, metal borides of Group 1A, Group 2A, Group 3A, and lanthanoids, and metal carbides of Group 4A, and a film thickness thereof is set to be not more than 100 .ANG.. PA1 wherein a material for forming the electron emission electrode is a material having a lower work function than a material for forming the electrode application electrode. PA1 wherein an oxide film is formed around the Schottky junction portion by an LOCOS method. PA1 sequentially depositing conductive layers serving as the semiconductor layer and the wiring electrode, the insulating layer, and a conductive layer serving as the lead electrode on the semiconductor substrate; forming a hole in the conductive layer serving as the lead electrode, the insulating layer, and the conductive layer serving as the wiring electrode; and performing ion implantation in the semiconductor layer through the hole to form a high-impurity concentration region. PA1 wherein the semiconductor layer of the first conductivity type has a high-concentration doping region of the first conductivity type, the high-concentration doping layer forming a Schottky junction with the Schottky electrode. PA1 a semiconductor substrate having a p-type semiconductor layer whose impurity concentration falls within a concentration range for causing an avalanche breakdown in at least a portion of a surface thereof, PA1 a Schottky electrode for forming a Schottky junction with the p-type semiconductor layer, PA1 means for applying a reverse bias voltage to the Schottky electrode and the p-type semiconductor layer to cause the Schottky electrode to emit electrons, and PA1 a lead electrode, formed at a proper position, for externally guiding the emitted electrons, PA1 the element comprising PA1 a semiconductor substrate having a p-type semiconductor layer whose impurity concentration falls within a concentration range for causing an avalanche breakdown in at least a portion of a surface thereof, PA1 a Schottky electrode for forming a Schottky junction with the p-type semiconductor layer, PA1 means for applying a reverse bias voltage to the Schottky electrode and the p-type semiconductor layer to cause the Schottky electrode to emit electrons, and PA1 a lead electrode, formed at a proper position, for externally guiding the emitted electrons, PA1 the element comprising PA1 wherein the Schottky electrode has a small thickness which is sufficient to pass electrons produced in a depletion layer of the Schottky junction in the avalanche breakdown state.
Therefore, a demand has arisen for an electron emission element which can use a material other than cesium or a cesium oxide.
In the prior art, hot electrons produced at a p-n interface lose their energies by scattering when they pass through an n-type semiconductor layer. In order to prevent this, the n-type semiconductor layer must be formed to be very thin (200 .ANG. or less). In order to uniformly form a very thin n-type semiconductor layer at a high concentration to be free from defects, there are many problems on semiconductor manufacturing processes which are incurred. Therefore, it is difficult to stably manufacture such an element in practice.
In an electron emission element in which a Schottky electrode is formed on the surface of a semiconductor layer, when the Schottky electrode is formed of a material having a low work function, the Schottky electrode is oxidized in the manufacturing process of the electron emission element to be denaturated into a high-resistance film or hydroxide. For this reason, the work function of the electron emission surface of the Schottky electrode is increased, resulting in poor electron emission efficiency and diode characteristics.
In the electron emission element described above with reference to FIG. 1, since the Schottky electrodes 108 and the lead electrodes 107 are formed after the high-impurity concentration region 103 is formed in the semiconductor layer 102, a position shift between the high-impurity concentration region 103 and the Schottky electrodes 108 or the lead electrodes 107 easily occurs. For this reason, an alignment margin must be increased to guarantee reliability or yield of the electron emission element. In terms of cost, an occupation area per element must often be increased.
In the method of manufacturing the electron emission element shown in FIG. 1, a photolithographic process must be repeated by a plurality of times corresponding to the number of times of ion implantation and the number of films to be deposited on the semiconductor layer 102. Therefore, the manufacturing process is complicated, resulting in high manufacturing cost.